1. Field of the Invention
The present invention relates to an image intensifier.
2. Related Background Art
FIG. 2 is a configurational view showing a typical proximity-type image intensifier.
The image intensifier shown in FIG. 2 is constituted, successively in a row from its light entrance surface side, by a photoelectric plate 1, an microchannel plate (MCP) 2, and an optical fiber block 4. An output phosphor face 3 is formed on the surface of the optical fiber block 4 on the entrance surface side. When light is incident on the entrance surface of the photoelectric plate 1, due to photoelectric conversion, an electron is emitted from the surface of the photoelectric plate 1 opposite to the entrance surface. This electron is accelerated and amplified by the MCP 2, and then impinges on the phosphor face 3, thus emitting fluorescent light. The emitted light is guided through the optical fiber block 4 to the output side, whereby the incident light is amplified so as to yield output light. In the proximity-type image intensifier, the phosphor face 3 and the MCP 2 are disposed close to each other, thereby enhancing image reproducibility while attaining a compact configuration.
In order to improve the output resolution of the image intensifier, various proposals have been made concerning the improvement of the output phosphor face. For example, one such proposal is disclosed in Japanese Patent Application Laid-Open No. 62-252043. According to the technique disclosed therein, as shown in FIG. 3, a core portion 42 on the entrance surface side of the optical fiber block 4 is formed with a pit 44 with respect to a cladding portion 43, and after the pit 44 is filled with a phosphor 45, a metal back layer 46 is formed on the surface on the entrance side.
In contrast to the case where a phosphor layer is uniformly applied to the whole surface of the optical fiber block on the entrance surface side, a higher ratio of light generated in the phosphor 45 is made incident on the core portion 42, and the loss caused by the light leaked into and absorbed by the cladding portion 43 can be reduced. This enhances transmission efficiency, while light from leaking into the adjacent core, thereby improving the resolution as well.
Nevertheless, the following problems may occur due to the fact that the electron is reflected on the output phosphor face. A part of the electrons accelerated from the MCP 2 are diffusely reflected by the surface of the metal back layer 46 of the phosphor face 3 when impinging thereon. FIG. 4 shows a state of this diffuse reflection. Between the MCP 2 and the phosphor face 3, an electric field is generated by virtue of a voltage applied thereto. As shown in FIG. 4, the path of the diffusely reflected electron is deflected use to this electric field. As such the electron may re-impinge on a wide area of the phosphor face 3.
Consequently, when a strong spot light enters the image intensifier, it allows a large amount of electrons to be incident on a narrow area of the phosphor face 3 from the MCP 2, and the electron, diffusely reflected on the phosphor face 3, may re-impinge on the phosphor face 3 over a much wider area than the initial incident area. As a result, a fluorescence effect may be generated in and outputted from an area wider than the incident area of the original spot light, thus deteriorating output resolution. This deterioration is commonly known as the halo phenomenon of output light.
This halo phenomenon can be reduced when the distance between the MCP 2 and the phosphor face 3 is decreased. Nevertheless, because there is a limit to the minimal distance therebetween, due to pressure resistance, it has conventionally been difficult to suppress the halo phenomenon. As such, output resolution with respect to strong spot light has been hard to improve.